Add docstrings everywhere!

Document modules.

Improve handling of invalid FontConfig data.

Fuzz test roundtrip conversions between C & Haskell datastructures!

Properly handle ambiguity in empty char/lang sets.

fuzz-test MessagePack implementations.

Constrain public API.

Encode & decode charsets on C side (fix Haskell side)

Datamodel FontConfig in Haskell.